The invention is directed to a method of the production of battery separators, particularly for lead-acid batteries of the type used in automobiles, for example. In batteries of the type referred to, a series of lead plates are disposed in an acid electrolyte, and separators are positioned in the spaces between the lead plates. The technical requirements of the separators are quite severe. Among other things, the separator material must be chemically inert to the plate material, the electrolyte, and the chemicals formed during the cycling of the battery. The separators, in addition, must be able to withstand the environment, both from a chemical standpoint and a physical standpoint. With respect to the latter, the separators, while permitting a free flow of ions in the electrolyte material, must prevent any electrical contact between adjacent plates, which might result in short circuiting.
All of these required functions must be carried out in an environment in which wide temperature variations are encountered, and in which the batteries are subjected to substantial vibration, acceleration and deceleration, and other physical movements encountered in normal vehicle use. Additionally, the separators must be capable of high volume production at low cost, in order to be economically attractive in the primary market, automobile batteries.
Various types of battery separators are known in the prior art, including separators constructed of paper, hard rubber, sintered polyvinylchloride, and the like. All of these various known types of separators have various advantages and disadvantages. Some, for example, while forming highly effective separators, are of such high cost as to be suitable only in special purpose applications. Others, while being of lower cost, are less effective in terms of performance under the exceptionally severe conditions required of an automobile battery.
The present invention deals particularly with a method of making battery separators formed of melt-blown thermoplastic web material. In general, such battery separators have been described in, for example, U.S. Pat. No. 3,881,957. However, notwithstanding the technical information available in the before mentioned patent and other disclosures both preceding and following it, I am not aware of any commercially successful battery separator formed of melt-blown thermoplastic material, other than that produced in accordance with the method of the present invention. In this respect, all of the objectives and all of the requirements of a good battery separator are well known to all concerned. Nevertheless, the manner of achieving the known requirements and at the same time being able to produce separators in high volume at acceptable cost levels remains elusive.
In accordance with the primary objective of the present invention, a novel and improved method is provided for the production of battery separators of melt-blown thermoplastic materials on a basis which is compatible with high volume, low cost requirements of the automotive market, yet which is capable of producing a battery separator of acceptably high quality standards.
In accordance with the process of the invention, a multiple ply web is formed of a plurality of individual webs of melt-blown thermoplastic resin material. In general, these individual webs are formed in accordance with known procedures, such as disclosed in the Buntin U.S. Pat. No. 3,849,241. A thermoplastic material, such as polypropylene, is heated and extruded through a series of extremely fine openings. The emerging fiber-like extrusions are subjected to the action of hot, high velocity air streams discharged from narrow slots immediately adjacent the extrusion die openings. A fibrous mat, of controlled properties, is collected on a moving, foraminous belt or drum. By controlling variables, such as the volume and/or temperature of the air stream, fibrous mats of different physical properties may be produced. In a multiple ply material for battery separator use, a core ply structure is formed of fine fibers, having extremely fine average pore size, while outer plies are formed of somewhat heavier fibers. Either in the original formation or subsequently, the multiple plies are bonded in the absence of any external adhesive material to form the desired composite material.
Pursuant to the invention, the melt-blown fibers of the multiple ply material are rendered wetable by battery electrolyte solution by coating with one or more compatible surfactant solutions or wetting agents a procedure which, in general, is well known. Importantly, after impregnation of the web with the surfactant solution, but before drying of the web, the multiple ply web is subjected to simultaneous heat and localized rolling pressure, in an operation effective to mechanically disperse the surfactant throughout the web and simultaneously to flash off a substantial portion of the surfactant solvent or carrier without, however, melting the thermoplastic material. Thereafter, the small residue of solvent or carrier may be removed from the web by conventional drying techniques, such as by passing hot air through the porous material.
To significant advantage, the operation of applying heat and localized rolling pressure is performed in a manner simultaneously to impart thickness compaction and longitudinal ribbing to the web material, to provide desired strength, rigidity and other characteristics to the web for the intended battery separator application.
The web material formed by the procedures described is converted to battery separators by being slit into appropriate width, cut to length and folded in half about the lower edge of a battery separator plate. The side edges of the folded separator material extend slightly beyond the edges of the plate and are thermoplastically bonded, providing an open top envelope for containment of the battery plate.
In one specifically advantageous modification of the new process, the multi-ply web material is formed in four stages, with the formation of four successive melt-blown webs, advantageously formed one upon the other in sequence. Pursuant to this aspect of the invention, an external web is formed, followed by two webs of core material, followed in turn by a second web of external material. While the end result does not differ significantly in terms of functional properties of the battery separator from a three-ply web, more advantageous utilization may be made of production capacity following the four web procedure.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment and to the accompanying drawing.